Optical sensor and method of use

ABSTRACT

An interferometer apparatus for an optical fibre system and method of use is described. The interferometer comprises an optical coupler and optical fibres which define first and second optical paths. Light propagating in the first and second optical paths is reflected back to the optical coupler to generate an interference signal. First, second and third interference signal components are directed towards respective first, second and third photodetectors. The third photodetector is connected to the coupler via a non-reciprocal optical device and is configured to measure the intensity of the third interference signal component directed back towards the input fibre. Methods of use in applications to monitoring acoustic perturbations and a calibration method are described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/322,448 filed on Mar. 12, 2012, which is a national stage applicationof PCT/GB2010/050888. Application PCT/GB2010/050888 was filed on May 27,2010, and claims priority to GB Application 0912051.0, filed on Jul. 11,2009, in the United Kingdom, and GB Application 0908990.5, filed on May27, 2009, in the United Kingdom. These applications are incorporated byreference in their entireties.

FIELD OF THE DISCLOSURE

The present disclosure relates to optical sensors and, in particular, toan interferometer and methods of use.

BACKGROUND

The benefits of optical fibers have been demonstrated in a number ofsensing applications. The two major areas are: (i) distributed opticalfiber sensors, and (ii) multiplexed point sensor arrays.

Distributed sensors utilise the intensity of backscatter light, withRaman and/or Brillouin peaks in the light signal utilised to measuretemperature, strain or pressure. Distributed sensors offer a number ofadvantages including continuous sensing along the entire length offiber, and flexibility and simplicity of the sensor, which may bestandard telecoms optical fiber. For example, a distributed sensor mayprovide 10,000 measurement points along 10 km of optical fiber with a 1m spatial resolution. Distributed sensor systems therefore offer lowinstallation and ownership costs.

However, due to their slow response, distributed sensors are usuallyonly used in applications where measurements taking in order of severalseconds to hours are acceptable. The most common sensors of this typeare the distributed temperature sensors (DTS), which are made by anumber of companies. A typical performance of a DTS is 1 m spatialresolution and 1° C. temperature resolution in 60 seconds over a 10 kmrange.

Distributed sensors have also been used to measure strain by utilisingBrillouin shifts in reflected or backscattered light, as described inU.S. Pat. No. 6,555,807 (Clayton et al.) or WO 98/27406 (Farhadiroushanet al.) The frequency of the Brillouin shift is about 1 MHz/10με and itslinewidth is about 30 MHz. The strain in an order of 10με can bedetermined along an optical fiber using the narrow frequency scanningmethods described. However, using these approaches, the scanning rate ismuch slower than the pulse repetition rate and measurement times aretypically in the order of few seconds to few minutes.

More recently, a technique for faster measurement of Brillouin frequencyshift has been proposed in U.S. Pat. No. 7,355,163 (Watley et al.). Thistechnique uses a frequency to amplitude convertor which may be in a formof an optical fiber Mach-Zehnder interferometer with a 3×3 coupler atits output. However, the strain resolution is limited by the linewidthof the Brillouin light and therefore the optical path length differencein the interferometer should be kept within the coherence length of theBrillouin light. Also, the polarisation fading between the two paths ofthe interferometer, the offset and gain variations of the photodetectorreceivers would significantly limit the strain measurement. Measurementtimes of around 0.1 seconds (10 Hz) with strain resolution of 50με havebeen recently reported using this technique.

For many applications, such as acoustic sensing, much highersensitivities and faster a measurement time in the order of 1millisecond (1 kHz), 0.1 millisecond (10 kHz) or 0.01 millisecond (100kHz) is required.

Multiplexed point sensors offer fast measurements with high sensitivityand are used, for example, in hydrophone arrays. The main applicationfor these in the energy market is for towed and seafloor seismic arrays.However, unlike with distributed sensors, multiplexed point sensorscannot be used where full coverage is required. The size and theposition of the sensing elements are fixed and the number of sensorsmultiplexed on a single fiber is typically limited to 50 to 100elements. Furthermore, the sensor design relies on additional opticalfiber components leading to bulky and expensive array architectures.There is also considerable effort to increase the number of sensors thatcan be efficiently multiplexed on a single length of fiber.

Optical-time-domain reflectometry (OTDR) is a well-known technique thathas been used to test optical fiber communications cables. In order toreduce the effect of coherent backscatter interference, which issometime is referred to as Coherent Rayleigh Noise, a broadband lightsource is normally used. However, proposals have also been made in U.S.Pat. No. 5,194,847 (Taylor et al.) to use coherent OTDR for sensingintrusion by detecting the fast changes in a coherent backscatterRayleigh signal. In addition, Shatalin et al. (Shatalin et al.“Interferometric optical time-domain reflectometry for distributedoptical-fiber sensing”, Applied Optics, Vol. e7, No. 24, pp. 5600-5604,20 Aug. 1998.) describes using coherent Rayleigh as a distributedoptical fiber alarm sensor.

WO 2008/056143 (Shatalin et al.) describes a disturbance sensor similarto that of U.S. Pat. No. 5,194,847 (Taylor et al.) using a semiconductordistributed feedback laser source. A fiber Bragg grating filter ofpreferably 7.5 GHz is used to reject out-of-band chirped light and,thereby, improve the coherence of the laser pulse sent into the fiber.However, this requires matching of the laser wavelength with the narrowband optical filter, which results in the signal visibility variationbeing reduced compared to a system which uses a very high coherentsource as proposed by U.S. Pat. No. 5,194,847.

Similar techniques have also been proposed for the detection of buriedoptical fiber telecommunication cables (for example in WO 2004/102840(Russel et al.)), in perimeter security (GB 2445364 (Strong et al.) andUS2009/0114386 (Hartog et al.)) and downhole vibration monitoring (WO2009/056855 (Hartog et al.)). However, the response of these coherentRayleigh backscatter systems has been limited by a number of parameterssuch as polarisation and signal fading phenomena; the random variationof the backscatter light; and non-linear coherent Rayleigh response.Therefore these techniques are mainly used for event detection and donot provide quantitative measurements, such as the measurement ofacoustic amplitude, frequency and phase over a wide range of frequencyand dynamic range.

SUMMARY OF INVENTION

The present disclosure provides novel apparatus and methods for fastquantitative measurement of perturbation of optical fields transmitted,reflected and or scattered along a length of an optical fiber.

Embodiments of the present disclosure can be used for distributedsensors, point sensors, or the combination of both.

In particular this technique can be applied to distributed sensors whileextending dramatically the speed and sensitivity to allow the detectionof acoustic perturbations anywhere along a length of an optical fiberwhile achieving fine spatial resolution. The present disclosure offersunique advantages in a broad range of acoustic sensing and imagingapplications. Typical uses are for monitoring oil and gas wells, forapplications such as for distributed flow metering and/or imaging;seismic imaging, monitoring long cables and pipelines; acoustic imaginginside large vessels as well as security applications.

Embodiments of the present disclosure provide apparatus for highlysensitive and fast quantitative measurement of the phase, frequency andamplitude of the light transmitted, reflected or scattered along alength of an optical fiber.

In the prior art, optical couplers have been used in Michelson orMach-Zehnder interferometer configurations where the polarisationbetween the two arms of the interferometer has to be carefullycontrolled. The novel interferometer in the present disclosure allows anm×m coupler to be utilised using non-reciprocal devices, such as Faradayrotator mirrors and an optical circulator, to provide compensated lightinterference with a given phase shift that can be measured at all portsof the optical coupler and analysed very quickly, such as at severaltens of kilohertz.

The embodiments of the disclosure can be used for multiplexed acousticpoint sensors, distributed sensors or a combination of both. In the caseof distributed sensors, light pulses are injected into the fiber and thephase modulation of the backscattered light is measured along the fiberat several tens of kilohertz. The fiber can be standardtelecommunication fiber and/or cable. Using the techniques describedherein, the sensing system can thereby detect the acoustic field alongthe fiber to provide a distributed acoustic sensor whereby the lengthsof the sensing elements can be selected by a combination of adjustingthe modulation of the light pulse, the path length in the interferometeras well as the sensing fiber configuration.

The data collected along the fiber are automatically synchronised andthey may be combined to provide coherent field images.

According to a first aspect of the disclosure, there is providedinterferometer apparatus for an optical fiber system, the apparatuscomprising: an optical coupler having an input port and first and secondports coupled to optical fibers which define first and second opticalpaths; first and second reflectors arranged respectively in the firstand second optical paths to reflect light propagating in the first andsecond optical paths back to the optical coupler to generate aninterference signal; wherein the optical coupler is configured to directfirst and second interference signal components respectively to firstand second detector ports, and is configured to direct a thirdinterference signal component towards the input port, and the apparatuscomprises means for introducing a phase shift between the first, secondand third interference signal components; first and secondphotodetectors connected to first and second detector ports of theoptical coupler and configured to measure an intensity of first andsecond interference signal components at respective phase shifts; andwherein the apparatus comprises a third photodetector connected to thenon-reciprocal optical device and configured to measure the intensity ofthe third interference signal component directed back towards the inputfiber.

The means for introducing a phase shift between the first, second andthird interference signal components may be the optical coupler,preferably an m×m optical coupler, where m>=3. The non-reciprocaloptical device may be an optical circulator.

The optical fibers and reflectors may be configured to maintainpolarisation or provide polarisation compensation for light propagatingin the first and second optical paths. The reflectors may be FaradayRotator Mirrors (FRMs), permitting the use of standard (non-polarisationmaintaining) fibers.

The non-reciprocal optical device may be configured to receive the lightsignal and transmit it to the input port of the optical coupler.

This arrangement provides an economical configuration of components,which allow all ports of the optical coupler to be used effectively. Thearrangement provides a “spare” port which may be used to cascademultiple interferometers together, or to couple to an additionaldetector or interferometer arm.

Other preferred and optional features of this aspect of the disclosureare defined by the claims. Furthermore, embodiments of this aspect ofthe disclosure may comprise preferred and optional features of otheraspects of the disclosure.

According to a second aspect of the disclosure there is provided aninterferometer system comprising a first interferometer apparatus asclaimed in any preceding claim, and a second interferometer apparatus asclaimed in any preceding claim, wherein a third output port of theoptical coupler of the first interferometer apparatus is coupled to aninput of the second interferometer apparatus.

The interferometer system may comprise multiple interferometerapparatuses, wherein respective output ports of a subset of theinterferometer apparatuses are utilised as inputs for sequentialinterferometer apparatuses.

The different interferometer apparatuses may have different optical pathlength differences. This facilitates selection of different spatialresolutions in applications of the interferometer system.

Other preferred and optional features of this aspect of the disclosureare defined by the claims. Furthermore, embodiments of this aspect ofthe disclosure may comprise preferred and optional features of otheraspects of the disclosure.

According to a third aspect of the disclosure there is provided anoptical fiber system for monitoring an optical signal, the systemcomprising: a light source; an optical fiber deployed in an environmentto be monitored and coupled to the light source; an interferometerapparatus as described in the first aspect and configured to receivebackscattered or reflected light from the optical fiber; data capturingmeans for gathering data output from the photodetectors of theinterferometer apparatus.

Other preferred and optional features of this aspect of the disclosureare defined by the claims. Furthermore, embodiments of this aspect ofthe disclosure may comprise preferred and optional features of otheraspects of the disclosure.

According to a fourth aspect of the disclosure there is provided amethod of monitoring acoustic perturbations, the method comprising:providing a light source, an optical fiber deployed in the environmentto be monitored and coupled to the light source, and an interferometerconfigured to receive a pulsed optical signal from the optical fiber,the interferometer comprising at least two optical paths and at leasttwo photodetectors; receiving backscattered or reflected light from theoptical fiber in the interferometer, and generating an interferencesignal; introducing a phase shift between first and second interferencesignal components of the interference signal, and directing the firstand second interference signal components to first and secondphotodetectors respectively; measuring the intensity of the first andsecond interference signal components at respective phase shifts toprovide first intensity data and second intensity data; processing thefirst and second intensity data to determine the optical phase angle ofthe optical signal and provide optical phase angle data; processing theoptical phase data to determine optical phase angle modulation data,and; identifying acoustic perturbations to which the optical fiber hasbeen exposed from the optical phase angle modulation data.

The step of identifying acoustic perturbations to which the opticalfiber has been exposed preferably comprises characterising the acousticperturbations.

The method may comprise generating an acoustic output signal from thecharacterised acoustic perturbations.

Other preferred and optional features of this aspect of the disclosureare defined by the claims. Furthermore, embodiments of this aspect ofthe disclosure may comprise preferred and optional features of otheraspects of the disclosure.

According to a fifth aspect of the disclosure there is provided a methodof operating an interferometer in an optical system, the methodcomprising: providing an interferometer comprising an input configuredto receive transmitted, reflected, or backscattered light from a firstlight source, at least first and second optical paths, and a pluralityof photodetectors; providing an incoherent light source configured toinput incoherent light to the interferometer; determining anormalisation factor for a photodetector offset, a relativephotodetector gain, and/or a coupling ratio of the interferometeroptical paths, by inputting light from an incoherent light source to theinterferometer and measuring the outputs of the photodetectors.

Other preferred and optional features of this aspect of the disclosureare defined by the claims. Furthermore, embodiments of this aspect ofthe disclosure may comprise preferred and optional features of otheraspects of the disclosure.

BRIEF DESCRIPTION OF DRAWINGS

Other preferred and optional features of this aspect of the disclosureare defined by the claims. Furthermore, embodiments of this aspect ofthe disclosure may comprise preferred and optional features of otheraspects of the disclosure.

FIGS. 1, 2, 3 and 4 show schematically novel interferometer apparatusaccording to related embodiments of the disclosure, comprisingcirculators and multiple fiber couplers with different optical pathsthrough the interferometers, Faraday-rotator mirrors and photodetectors.

FIGS. 5 and 6 show schematically how the interferometers can be cascadedaccording to embodiments of the disclosure in series and/or starconfigurations.

FIG. 7 shows schematically a sensor system that utilises theinterferometer of an embodiment of the disclosure for fast measurementof scattered and reflected light from an optical fiber.

FIG. 8 shows schematically a distributed sensor system that utilises theinterferometer of an embodiment of the disclosure to generate a seriesof pulses each of different frequency.

FIG. 9 is a block diagram representing a data processing methodaccording to an embodiment of the disclosure.

FIG. 10 is a block diagram representing a method of calibrating theinterferometer according to an embodiment of the disclosure.

FIG. 11 shows schematically an embodiment in which the fiber can bedeployed as linear sensors, directional sensors or in a multidimensionalarray of sensors.

DETAILED DESCRIPTION

FIG. 11 shows schematically an embodiment in which the fiber can bedeployed as linear sensors, directional sensors or in a multidimensionalarray of sensors.

FIG. 1 shows a first embodiment, generally depicted at 100, of a novelinterferometer for measuring the optical amplitude, phase and frequencyof an optical signal. The incoming light from a light source (not shown)is preferably amplified in an optical amplifier 101, and transmitted tothe optical filter 102. The filter 102 filters the out of band AmplifiedSpontaneous Emission noise (ASE) of the amplifier 101. The light thenenters into an optical circulator 103 which is connected to a 3×3optical coupler 104. A portion of the light is directed to thephotodetector 112 to monitor the light intensity of the input light. Theother portions of light are directed along first and second opticalpaths 105 and 106, with a path length difference between the two paths.Faraday-rotator mirrors (FRMs) 107 and 108 reflect the light backthrough the first and second paths 105 and 106, respectively. TheFaraday rotator mirrors provide self-polarisation compensation alongoptical paths 105 and 106 such that the two portions of lightefficiently interfere at each of the 3×3 coupler 104 ports. The opticalcoupler 104 introduces relative phase shifts of 0 degrees, +120 degreesand −120 degrees to the interference signal, such that first, second andthird interference signal components are produced, each at a differentrelative phase.

First and second interference signal components are directed by theoptical coupler 104 to photodetectors 113 and 114, which measure theintensity of the respective interference signal components.

The circulator 103 provides an efficient path for the input light andthe returning (third) interference signal component through the sameport of the coupler 104. The interference signal component incident onthe optical circulator 103 is directed towards photodetector 115 tomeasure the intensity of the interference signal component.

The outputs of the photodetectors 113, 114 and 115 are combined tomeasure the relative phase of the incoming light, as described in moredetail below with reference to FIGS. 7 and 9.

Optionally, frequency shifters 110 and 111 and/or optical modulator 109may be used along the paths 105 and 106 for heterodyne signalprocessing. In addition, the frequency shift of 110 and 111 may bealternated from f1, f2 to f2, f1 respectively to reduce anyfrequency-dependent effect between the two portions of the lightpropagating through optical paths 105 and 106.

The above-described embodiment provides a novel apparatus suitable forfast quantitative measurement of perturbation of optical fields, and inparticular can be used for distributed and multiplexed sensors with highsensitivity and fast response times to meet requirements of applicationssuch as acoustic sensing.

FIG. 7 shows an application of the interferometer of FIG. 1 to thedistributed sensing of an optical signal from an optical system 700. Itwill be apparent that although the application is described in thecontext of distributed sensing, it could also be used for point sensing,for example by receiving reflected light from one or more point sensorscoupled to the optical fiber.

In this embodiment 700, light emitted by a laser 701 is modulated by apulse signal 702. An optical amplifier 705 is used to boost the pulsedlaser light, and this is followed by a band-pass filter 706 to filterout the ASE noise of the amplifier. The optical signal is then sent toan optical circulator 707. An additional optical filter 708 may be usedat one port of the circulator 707. The light is sent to sensing fiber712, which is for example a single mode fiber or a multimode fiberdeployed in an environment in which acoustic perturbations are desiredto be monitored. A length of the fiber may be isolated and used as areference section 710, for example in a “quiet” location or with acontrolled reference signal. The reference section 710 may be formedbetween reflectors or a combination of beam splitters and reflectors 709and 711.

The reflected and the backscattered light generated along the sensingfiber 712 is directed through the circulator 707 and into theinterferometer 713. The detailed operation of the interferometer 713 isdescribed earlier with reference to FIG. 1. In this case, the light isconverted to electrical signals using fast low-noise photodetectors 112,113, 114 and 115. The electrical signals are digitized and then therelative optical phase modulation along the reference fiber 710 and thesensing fiber 712 is computed using a fast processor unit 714 (as willbe described below). The processor unit is time synchronized with thepulse signal 702. The path length difference between path 105 and path106 defines the spatial resolution. The photodetector outputs may bedigitized for multiple samples over a given spatial resolution. Themultiple samples are combined to improve the signal visibility andsensitivity by a weighted averaging algorithm combining thephotodetector outputs.

Data Processing

FIG. 9 schematically represents a method 900 by which the optical phaseangle is determined from the outputs of the photodetectors 113, 114,115. The path length difference between path 105 and path 106 definesthe spatial resolution of the system. The photodetector outputs may bedigitized for multiple samples over a given spatial resolution, i.e. theintensity values are oversampled. The multiple samples are combined toimprove the signal visibility and sensitivity by a weighted averagingalgorithm combining the photo-detector outputs.

The three intensity measurements I₁ I₂, I₃, from the photodetectors 113,114, 115 are combined at step 902 to calculate the relative phase andamplitude of the reflected or backscattered light from the sensingfiber. The relative phase is calculated (step 904) at each samplingpoint, and the method employs oversampling such that more data pointsare available than are needed for the required spatial resolution of thesystem. Methods for calculating the relative phase and amplitude fromthree phase shifted components of an interference signal are known fromthe literature. For example, Zhiqiang Zhao et al. (Zhao et al. “ImprovedDemodulation Scheme for Fiber Optic Interferometers Using an Asymmetric3×3 Coupler”, J Lightwave Technology, Vol. 13, No. 11, November 1997,pp. 2059-2068) and U.S. Pat. No. 5,946,429 (Huang et al.) describetechniques for demodulating the outputs of 3×3 couplers in continuouswave multiplexing applications. The described techniques can be appliedto the time series data of the present embodiment.

For each sampling point, a visibility factor V is calculated at step 906from the three intensity measurements I₁, I₂, I₃, from thephotodetectors 113, 114, 115, according to equation (1), for each pulse.

V=(I ₁ −I ₂)²+(I ₂ −I ₃)²+(I ₃ −I ₁)²  Equation (1)

At a point of low visibility, the intensity values at respective phaseshifts are similar, and therefore the value of V is low. Characterizingthe sampling point according the V allows a weighted average of thephase angle to be determined (step 908), weighted towards the samplingpoints with better visibility. This methodology improves the quality ofthe phase angle data 910.

Optionally, the visibility factor V may also be used to adjust (step912) the timing of the digital sampling of the light for the maximumsignal sensitivity positions. Such embodiments include a digitizer withdynamically varying clock cycles, (which may be referred to herein as“iclock”). The dynamically varying clock may be used to adjust thetiming of the digitized samples at the photodetector outputs for theposition of maximum signal sensitivity and or shifted away frompositions with poorer visibility.

The phase angle data is sensitive to acoustic perturbations experiencedby the sensing fiber. As the acoustic wave passes through the opticalfiber, it causes the glass structure to contract and expand. This variesthe optical path length between the backscattered light reflected fromtwo locations in the fiber (i.e. the light propagating down the twopaths in the interferometer), which is measured in the interferometer asa relative phase change. In this way, the optical phase angle data canbe processed at 914 to measure the acoustic signal at the point at whichthe light is generated.

In preferred embodiments of the disclosure, the data processing method900 is performed utilizing a dedicated processor such as a FieldProgrammable Gate Array.

Sensor Calibration

For accurate phase measurement, it is important to measure the offsetsignals and the relative gains of the photo-detectors 113, 114 and 115.These can be measured and corrected for by method 1000, described withreference to FIG. 10.

Each photodetector has electrical offset of the photodetectors, i.e. thevoltage output of the photodetector when no light is incident on thephotodetector (which may be referred to as a “zero-light level” offset.As a first step (at 1002) switching off the incoming light from theoptical fiber and the optical amplifier 101. When switched off, theoptical amplifier 101 acts as an efficient attenuator, allowing nosignificant light to reach the photodetectors. The outputs of thephotodetectors are measured (step 1004) in this condition to determinethe electrical offset, which forms a base level for the calibration.

The relative gains of the photodetectors can be measured, at step 1008,after switching on the optical amplifier 101 while the input light isswitched off (step 1006). The in-band spontaneous emission (i.e. theAmplified Spontaneous Emission which falls within the band of thebandpass filter 102), which behaves as an incoherent light source, canthen be used to determine normalization and offset corrections (step1010) to calibrate the combination of the coupling efficiency betweenthe interferometer arms and the trans-impedance gains of thephotodetectors 113, 114 and 115. This signal can also be used to measurethe signal offset, which is caused by the in-band spontaneous emission.

Conveniently, the optical amplifier, which is a component of theinterferometer, is used as an incoherent light source without arequirement for an auxiliary source. The incoherence of the source isnecessary to avoid interference effects at the photodetectors, i.e. thecoherence length of the light should be shorter than the optical pathlength of the interferometer. However, for accurate calibration it ispreferable for the frequency band of the source to be close to, orcentred around, the frequency of light from the light source. Thebandpass filter 102 is therefore selected to filter out light withfrequencies outside of the desired bandwidth from the AmplifiedSpontaneous Emission.

When used in a pulsed system, such as may be used in a distributedsensor, the above-described method can be used between optical pulsesfrom the light source, to effectively calibrate the system during use,before each (or selected) pulses from the light source withsubstantively no interruption to the measurement process.

Variations to the above-described embodiments are within the scope ofthe disclosure, and some alternative embodiments are described below.FIG. 2 shows another embodiment, generally depicted at 200, of a novelinterferometer similar to that shown in FIG. 1 but with an additionalFaraday-rotator mirror 201 instead of photodetector 112. Like componentsare indicated by like reference numerals. In this case the interferencebetween different paths, which may have different path length, can beseparated at the three beat frequencies f₁, f₂ and (f₂−f₁). Thearrangement of this embodiment has the advantage of providing additionalflexibility in operation, for example the different heterodynefrequencies can provide different modes of operation to generatemeasurements at different spatial resolutions.

FIG. 3 shows another embodiment of a novel interferometer, generallydepicted at 300, similar to the arrangement of FIG. 1, with likecomponents indicated by like reference numerals. However, thisembodiment uses a 4×4 coupler 314 and an additional optical path 301,frequency shifter 304, phase modulator 303, Faraday-rotator mirror 302and additional photo-detector 308. In this case the interference betweendifferent paths, which may have different path length differences, canbe separated at the three beat frequencies (f₂−f₁), (f₃−f₂) and (f₃−f₁).Alternatively, the Faraday-rotator mirror 302 may be replaced by anisolator or a fiber matched end so that no light is reflected throughpath 301, so only allowing interference between path 105 and 106.

An m×m coupler that generates m interference signal components atdifferent relative phase shifts may also be used in other embodiments ofthe disclosure.

FIG. 4 shows another embodiment of the interferometer. In this case anadditional path is introduced in the interferometer by inserting aFaraday-rotator mirror 402 instead of the photo-detector 112.

In all of the above-described embodiments, optical switches may be usedto change and/or select different combinations of optical path lengthsthrough the interferometer. This facilitates switching between differentspatial resolution measurements (corresponding to the selected pathlength differences in the optical path lengths).

FIGS. 5 and 6 show examples of interferometer systems 500, 600 arrangedfor used in cascaded or star configurations to allow the measuring ofthe relative optical phase for different path length differences. InFIG. 5, three interferometers 501, 502, 503 having different path lengthdifferences (and therefore different spatial resolutions) are combinedin series. In FIG. 6, four interferometers 602, 603, 604 and 605 havingdifferent path length differences (and therefore different spatialresolutions) are combined with interferometers 602, 603, 604 inparallel, and interferometers 603 and 605 in series. In FIG. 6, 601 is a3×3 coupler, used to split the light between the interferometers.Arrangement 600 can also be combined with wavelength divisionmultiplexing components to provide parallel outputs for differentoptical wavelengths.

FIG. 11 shows an embodiment with distributed sensors with the sensingfiber 702 subjected to different perturbation fields 1102, 1104 and1107. The sensing fiber can be used as linear sensors 1103 and 1104, asdirectional sensors 1105 and 1106 or as multi-dimensional array sensors1108, 1109 and 1110. Since all the measurements are synchronized, theycan be processed to enhance the signal sensitivity, achieve a widedynamic range and provide field imaging using beam forming techniques.

The embodiments described with reference to FIGS. 1 to 7 and 9 to 11relate to apparatus and methods for fast quantitative measurement ofacoustic perturbations of optical fields transmitted, reflected and orscattered along a length of an optical fiber. Embodiments of thedisclosure in its various aspects can be applied or implemented in otherways, for example to monitor an optical signal generated by a laser,and/or to monitor the performance of a heterodyne signal generator, andto generate optical pulses for transmission into an optical signal. Anexample is described with reference to FIG. 8.

FIG. 8 shows a system, generally depicted at 800, comprising aninterferometer 801 in accordance with an embodiment of the disclosure,used to generate two optical pulses with one frequency-shifted relativeto the other. The interferometer receives an input pulse from a laser701, via optical circulator 103. A 3×3 optical coupler 104 directs acomponent of the input pulse to a photodetector, and components to thearms of the interferometer. One of the arms includes a frequency shifter110 and an RF signal 805. The interference between the two pulses ismonitored by a demodulator 802. The light reflected by Faraday-rotatormirrors 107 and 108 is combined at the coupler 809 using a delay 803 tomatch the path length of the interferometer, so that the frequencyshifted pulse and the input pulse are superimposed. The coupler 809introduces relative phase shifts to the interference signal, andinterferometer therefore monitors three heterodyne frequency signalcomponents at relative phase shifts. The optical circulator 103 passesthe two pulses into the sensing fiber.

Review of Features of the Disclosure in its Various Aspects andEmbodiments

In one aspect, the disclosure provides an optical interferometerapparatus which can provide multiple path differences between theoptical signals and provide interference signals between differentoptical paths with fixed and/or variable phase shifts. Theinterferometer utilizes beam splitting components, circulating devicesand Faraday rotator mirrors in a novel configuration. The opticalsignals at the output of the interferometer are converted to electricalsignals which digitized for fast processing. The offset levels of theelectrical signals are removed and their amplitude are normalized. Therelative phase shifts of optical signals are accurately determined bycombining the normalized electrical signals.

In another aspect, the disclosure relates to an interferometer apparatusthat utilizes beam splitters and non-reciprocal devices to provide lightinterference with given phase shifts and path length differences thatcan be measured at all ports of the beam splitters whereby the relativephase modulation of the light can be computed very accurately andquickly, such as at every few nanoseconds. The interferometer may useoptical fiber components such as an m×m fused optical fiber coupler thatis connected to an optical fiber circulator at one of its ports;Faraday-rotator mirrors that reflect and, at the same time, providepolarization compensation for the light propagating through thedifferent paths of the interferometer and photodetectors that are usedto measure the interference light signals. The incoming optical lightmay be amplified using an optical fiber amplifier, and preferably theinterferometer has a pass band optical filter to filter out the out ofband Amplified Spontaneous Emission noise (ASE). The interferometer mayprovide birefringence compensation for light propagating along differentoptical paths through the interferometer. This provides sufficientlyhigh visibility at the outputs of the interferometer.

In another of its aspects, the disclosure provides a method forcompensating the offset and the gain of the photo-detectors, and thecoupling ratio of the interferometer arms, to normalize the resultantinterference signals used to measure the relative phase of the modulatedinput light in any of preceding claims where the detector offset ismeasured by switching off the optical amplifier in the backscatter path;the resultant photo-detector offset and gain then being determined byswitching on the amplifier while the input light is switched off; theASE of the optical amplifier then acts as an independent incoherentlight source and thereby the offsets and relative gains of thephoto-detectors can be determined and the detected light signalsnormalized. The method may therefore use incoherent light that entersthe input of the interferometer to normalize the relative signalamplitudes at the output of the photo-detectors. For example, when anoptical preamplifier is used at the input of the interferometer, thespontaneous light emission can be used to measure the combination of thesplitting ratio of the interferometer arms and the relative gains of thephoto-detectors and thereby normalize the relative signal amplitudesaccordingly.

Another additional feature of the present disclosure is to use phasemodulators and/or frequency shifters to shift the relative frequency andor vary the phase between the optical paths of the interferometer.Frequency shifters and/or phase modulators may be used to provideheterodyne signals and/or to separate the resultant interference lightsignal from different paths through the interferometer.

An additional feature of an embodiment of the disclosure is selectingthe frequency of the frequency shifter sufficiently high so that atleast one cycle of the beat frequency results within one light pulseresolution. Different frequency shifts may be used between differentoptical paths of the interferometer for the separation and/or heterodynedetection of the phase between different optical paths. The frequencyshifts between different optical paths may be alternated to correct forany frequency dependency of the interferometer output signals.

An additional feature of an embodiment of the disclosure is theselection of different optical paths through the interferometer such asby using optical switches. The optical switches may be used to selectdifferent optical paths through the interferometer and thereby select adifferent spatial resolution measurement. Another aspect of thedisclosure relates to a system comprising a number of interferometerscascaded in a series or in a star configuration or a combination ofboth.

The disclosure also provides a system that utilizes a light pulse formultiplexed and/or distributed sensors by measuring the phase modulationof the reflected and/or the backscattered light along a length of fiberwith high sensitivity, high dynamic range and a high speed of over tensof kilohertz. In this way, the disclosure can provide a multiplexedand/or distributed acoustic sensing system.

An additional feature of an embodiment of the disclosure is digitizingthe outputs of the interferometer, or the photodetectors of theinterferometer, at least twice over a spatial resolution interval. Anadditional feature of an embodiment of the disclosure is combining theoutputs of the interferometer to determine the insensitive measurementsample points resulting from any signal fading of the light in order toreject and/or provide a weighted signal average of the multiple samplesof the light over a given spatial resolution measurement or interval.Embodiments of the disclosure use a digitizer with dynamically varyingclock cycles, (which may be referred to herein as “iclock”), to adjustthe timing of the digital sampling of the light for the maximum signalsensitivity positions. The dynamically varying clock may be used toadjust the timing of the digitized samples at the photo-detector outputsfor the position of maximum signal sensitivity and or shifted away wherelight signal fading occurs.

Embodiments of the disclosure may use a laser light or a broadband lightsource. Coherent matching of the light with the same delay results in aninterference signal that can be used to measure the relative phasemodulation of the scattered or reflected light along the fiber.Embodiments of the disclosure may use wavelength division multiplexedcomponents to utilize multiple laser light pulses with differentwavelengths and, preferably, varying time shift with respect to each tocontrol the cross-phase modulation between the light pulses and to allowthe processing of multiple pulses in the sensing fiber without andcross-sensitivity to allow the system to achieve a higher measurandfrequency response. This may be the acoustic frequency response of thesystem to provide a different spatial sampling resolutions and/orpositions, and/or to allow the efficient rejection of any points withlow sensitivity.

An additional feature of an embodiment of the disclosure is theselection of different spatial resolutions whereby the sensitivity andthe frequency response along the sensing fiber can be adjusted, and thedynamic range can be widened.

The sensing fiber may be single mode fiber, polarization maintainingfiber, a single polarization fiber, multimode fiber, and/or a ribbonfiber, and it may be coated and/or cabled to enhance or to suppress itssensitivity.

An additional feature of an embodiment of the disclosure is theselection of different configurations of the fiber to optimize thesensitivity, the frequency and the directionality of the sensing fiberat different locations. The fiber may be deployed as linear sensors,direction sensors or multidimensional array sensors. The fiber may beplaced on a surface area in a continuous path without crossing overanother part of the fiber to increase the sensitivity.

The fiber may be attached on a surface of a vessel to listen to thenoise generated within the vessel to monitor the changes in the process,acoustically image the process, as well to detect any leaks.

A further aspect provides an apparatus using acoustic sensors fordistributed flow measurement and imaging, in-well perforated zonesmonitoring and sand production monitoring. For example, for in-wellapplications, the acoustic noise profile can be used to measure the flowby noise logging at every location along the well. In addition, thenoise spectrum can be used to identify the phase of the fluid. Furthernoise spectrum correlation techniques can be used over a long section ofthe well to determine the speed of sound as well as tracking eddiesgenerated within the flow to accurately determine the flow rates.

The sensor systems may be used as a distributed acoustic sensor,enabling the determination of distributed flow measurement and imaging,perforated zones monitoring and sand production monitoring in oil andgas wells and flowlines. The distributed temperature and strainmeasurements may be combined to enhance the data interpretation of thedistributed acoustic sensor.

Another aspect provides pipeline monitoring apparatus where the sensingfiber is deployed inside the pipeline and carried along the pipeline bythe fluid drag to provide a measurement of the noise flow fordiagnostics of the pipeline as well as for flow characterization and/orimaging.

Other advantages and applications of the disclosure will be apparent tothose skilled in the art. Any of the additional or optional features canbe combined together and combined with any of the aspects, as would beapparent to those skilled in the art.

As has been described above, apparatus and methods for fast quantitativemeasurement of perturbations of optical fields transmitted, reflectedand/or scattered along a length of an optical fiber. In particular,embodiments of the disclosure can be used for distributed sensing whileextending dramatically the speed and sensitivity to allow the detectionof acoustic perturbations anywhere along a length of an optical fiberwhile achieving fine spatial resolution. Embodiments of the presentdisclosure offer unique advantages in a broad range of acoustic sensingand imaging applications. Typical uses are for monitoring oil and gaswells such as for distributed flow metering and/or imaging, monitoringlong cables and pipelines, imaging of large vessels as well as securityapplications.

There follows a set of features describing particular embodiments of thedisclosure. The features below may be considered in combination unlessotherwise indicated.

An interferometer apparatus for an optical fibre system may comprise anoptical coupler having an input port and first and second ports coupledto optical fibres which define first and second optical paths, first andsecond reflectors arranged respectively in the first and second opticalpaths to reflect light propagating in the first and second optical pathsback to the optical coupler to generate an interference signal, whereinthe optical coupler is configured to direct first and secondinterference signal components respectively to first and second detectorports, and is configured to direct a third interference signal componenttowards the input port, and the apparatus comprises means forintroducing a phase shift between the first, second and thirdinterference signal components. The apparatus also includes first andsecond photodetectors connected to first and second detector ports ofthe optical coupler and configured to measure an intensity of first andsecond interference signal components at respective phase shifts. Theapparatus may further comprise a third photodetector connected to thenon-reciprocal optical device and configured to measure the intensity ofthe third interference signal component directed back towards the inputfibre.

The means for introducing a phase shift between the first, second andthird interference signal components of the interferometer apparatus maybe the optical coupler.

The optical coupler of the interferometer apparatus may be an m×moptical coupler, where m>=3.

The non-reciprocal optical device of the interferometer apparatus may bean optical circulator.

The optical fibres and reflectors of the interferometer apparatus may beconfigured to maintain polarisation or provide polarisation compensationfor light propagating in the first and second optical paths. Thereflectors of the interferometer apparatus may be Faraday RotatorMirrors (FRMs).

The interferometer apparatus described above may further comprise anoptical amplifier configured to receive an input light signal and outputan amplified light signal to the optical coupler.

The interferometer apparatus described above may further comprise a bandpass filter configured to filter out of band Amplified SpontaneousEmission noise (ASE) generated by the amplifier. According to someembodiments, the non-reciprocal optical device of the interferometerapparatus may be configured to receive the amplified light signal andtransmit it to the input port of the optical coupler.

A third output port of the optical coupler of the interferometerapparatus described above may be connected to a fourth photodetector.

The interferometer apparatus as described above may further comprise athird output port of the optical coupler, wherein the third output portis coupled to an optical fibre defining a third optical path, and theapparatus comprises a third reflector arranged in the third optical pathto reflect light propagating in the third optical path back to theoptical coupler to generate an interference signal.

The interferometer apparatus as described above may further comprise afrequency shifter or optical modulator in at least one of the opticalpaths.

The interferometer apparatus as described above may further comprise afrequency shifter or optical modulator in the first and second opticalpaths.

The interferometer apparatus as described above may further comprise anoptical switch configured to permit selection of different opticalpaths.

An interferometer system comprising a first interferometer apparatus asdescribed above, and a second interferometer apparatus as describedabove, wherein a third output port of the optical coupler of the firstinterferometer apparatus is coupled to an input of the secondinterferometer apparatus.

The interferometer system may comprise multiple interferometerapparatuses, wherein respective output ports of a subset of theinterferometer apparatuses are utilised as inputs for sequentialinterferometer apparatuses. According to some embodiments, theinterferometer system may include the interferometer apparatusescascaded in series and/or in a star configuration. The interferometersystem with the interferometer apparatuses cascaded in series and/or ina star configuration may be configured such that differentinterferometer apparatuses have different optical path lengthdifferences.

An optical fibre system for monitoring an optical signal may comprise alight source, an optical fibre deployed in an environment to bemonitored and coupled to the light source; an interferometer apparatusof any of features described above and configured to receivebackscattered or reflected light from the optical fibre, and a datacapturing means for gathering data output from the photodetectors of theinterferometer apparatus.

The optical fibre system may further comprise data processing apparatusfor processing the data output from the photodetectors to derive anacoustic signal.

The optical fibre system described above may be configured such that theoptical fibre comprises a fibre selected from the group comprising:multimode fibre, single mode fibre, polarisation maintaining fibre, asingle polarisation fibre, and/or a ribbon fibre.

The optical fibre system of any of features described above may beconfigured such that the optical fibre is coated and/or incorporated ina cable to enhance or suppress its sensitivity to acousticperturbations.

The optical fibre system described above may be configured such that theenvironment to be monitored is subterranean. The environment to bemonitored may also comprise a wellbore.

The optical fibre system described above may be configured such that theenvironment to be monitored comprises a pipeline.

The optical fibre system described above may be configured such that theoptical fibre is deployed linearly. In another embodiment, the opticalfibre system described above may be configured such that the opticalfibre is deployed at least partially in a planar arrangement to providedirectional sensing.

The optical fibre system described above may be configured such that theoptical fibre is deployed in a multidimensional array.

The optical fibre system described above may be configured such that atleast a part of the optical fibre is arranged on a surface area in acontinuous path without crossing over another part of the fibre.According to such an embodiment, the optical fibre system may beconfigured such that at least a part of the optical fibre is arranged ina folded three-Omega ([Omega] [Omega] [Omega]) configuration. Theoptical fibre system of feature 29 or feature 30 wherein at least a partof the optical fibre is arranged in a double-eight (88) configuration.

A method of monitoring acoustic perturbations includes providing a lightsource, an optical fibre deployed in the environment to be monitored andcoupled to the light source, and an interferometer configured to receivea pulsed optical signal from the optical fibre, the interferometercomprising at least two optical paths and at least three photodetectors.The method also includes receiving backscattered or reflected light fromthe optical fibre in the interferometer, and generating an interferencesignal and introducing a phase shift between first, second and thirdinterference signal components of the interference signal, and directingthe first, second and third interference signal components to first,second and third photodetectors respectively. The intensity of thefirst, second and third interference signal components are measured atrespective phase shifts to provide first intensity data, secondintensity data, and third. The method further includes processing thefirst, second and third intensity data to determine the optical phaseangle of the optical signal to thereby provide optical phase angle dataand optical phase angle modulation data and identifying acousticperturbations to which the optical fibre has been exposed from theoptical phase angle modulation data.

The step of identifying acoustic perturbations to which the opticalfibre has been exposed may further comprise characterising the acousticperturbations. The method may further comprise generating an acousticoutput signal from the characterised acoustic perturbations.

The method described above may further comprise frequency shifting orphase modulating light propagating in the interferometer to provide aheterodyne signal. The method further comprising frequency shifting mayinclude frequency shifting light propagating in the first optical pathby a first frequency U and frequency shifting light propagating in thesecond optical path by a second frequency f2 during a first mode ofoperation; and frequency shifting light propagating in the first opticalpath by a frequency different from U and frequency shifting lightpropagating in the second optical path by a frequency different from f2during a second mode of operation. In some embodiments, in the secondmode of operation, the light propagating in the first optical path isfrequency shifted by f2 and light propagating in the second optical pathis frequency shifted by fi.

In some embodiments, the interferometer comprises a third optical path,and the method comprises frequency shifting or phase modulating lightpropagating in at least one of the optical paths to allow separation ofthe interference generated from a recombination of light from therespective different paths through the interferometer.

The method described above may further comprise frequency shifting orphase modulating light propagating in the interferometer to provideheterodyne signals from the first, second and third optical paths.

The method described above may further comprise frequency shifting lightpropagating in the interferometer by a frequency sufficiently high toproduce a cycle of the heterodyne beat signal within a single pulse ofthe optical signal.

The method may further comprise selecting a spatial resolution intervalat which optical phase angle data is desired, and oversampling theoutputs of the photodetectors to provide multiple optical phase angledata over the spatial resolution interval. According to such a method,the outputs of the photodetectors may be sampled at least twice over thespatial resolution interval. The method may further comprise determininga visibility factor from the combined outputs of the photodetectors ateach sampled point, and providing a weighted signal average of opticalphase angle data from multiple sample points over the spatial resolutioninterval in dependence on the visibility factor. According to someembodiments, the method may further comprise adjusting the timing of thedigitised samples of the photodetector outputs. According to someembodiments the method may further comprise determining a visibilityfactor from the combined outputs of the photodetectors at each sampledpoint; and adjusting the timing of the digitised samples of thephotodetector outputs in dependence on the visibility factor. Accordingto some embodiments, the visibility factor may be calculated bycombining the squares of the differences of the intensity measurementsfrom each photodetector.

The method described above may be configured such that the light sourceis a laser light or a broadband light source.

The method described above may further comprise determining anormalisation factor for a photodetector offset, a relativephotodetector gain, and/or a coupling ratio of the interferometeroptical paths, by inputting light from an incoherent light source to theinterferometer and measuring the outputs of the photodetectors.According to such an embodiment, the method may further comprisefiltering the light from the incoherent light source using a bandpassfilter, such that the light input to the interferometer has a bandwidtharound the frequency of light transmitted, reflected or backscatteredlight propagating through the fibre. According to the embodiments ofthis paragraph, the light input to the interferometer may have acoherence length shorter than the optical path length of theinterferometer such that substantially no interference signal isdetected.

The method including determining a normalisation factor for aphotodetector offset, a relative photodetector gain, and/or a couplingratio of the interferometer optical paths may further comprisedetermining an electrical photodetector offset for each of thephotodetectors from the outputs of the photodetectors in a firstcondition, in which transmitted, reflected or backscattered lightpropagating through the fibre is decoupled from the interferometer andthe incoherent light source is switched off so that no light signal isinput to the interferometer; and determining a photodetector offset, arelative photodetector gain, and/or a coupling ratio of theinterferometer optical paths from the outputs of the photodetectors in asecond condition in which transmitted, reflected or backscattered lightpropagating through the fibre is decoupled from the interferometer andthe incoherent light source is switched on to input light to theinterferometer.

The method including determining a normalisation factor for aphotodetector offset, a relative photodetector gain, and/or a couplingratio of the interferometer optical paths may further comprise providingan optical amplifier configured to receive transmitted, reflected orbackscattered light propagating through the fibre and output anamplified light signal to the interferometer; and utilising theAmplified Spontaneous Emission of the optical amplifier (ASE) as theincoherent light source. According to the embodiment described in thisparagraph, the method may further comprise operating the interferometerin a third condition, in which the optical amplifier receives an inputlight signal from the light source and outputs an amplified light signalto the interferometer; and correcting for offsets and normalising thesignals detected at the photodetectors using the determinednormalisation factor. In some embodiments, the method may furthercomprise determining a normalisation factor and correcting for offsetsand normalising the signals detected at the photodetectors betweenoptical signal pulses.

A method of operating an interferometer in an optical system includesproviding an interferometer comprising an input configured to receivetransmitted, reflected, or backscattered light from a first lightsource, at least first and second optical paths, and a plurality ofphotodetectors; providing an incoherent light source configured to inputincoherent light to the interferometer; determining a normalisationfactor for a photodetector offset, a relative photodetector gain, and/ora coupling ratio of the interferometer optical paths, by inputting lightfrom an incoherent light source to the interferometer and measuring theoutputs of the photodetectors. This method may further comprisefiltering the light from the incoherent light source using a bandpassfilter, such that the light input to the interferometer has a bandwidtharound the frequency of light from the first light source. In someembodiments, the light input to the interferometer has a coherencelength shorter than the optical path length of the interferometer suchthat substantially no interference signal is detected.

The method of operating an interferometer in an optical system mayfurther comprise determining an electrical photodetector offset for eachof the photodetectors from the outputs of the photodetectors in a firstcondition, in which the first light source is decoupled from theinterferometer and the incoherent light source is switched off so thatno light signal is input to the interferometer; and determining aphotodetector offset, a relative photodetector gain, and/or a couplingratio of the interferometer optical paths from the outputs of thephotodetectors in a second condition in which the first light source isdecoupled from the interferometer and the incoherent light source isswitched on to input light to the interferometer.

The method of operating an interferometer in an optical system mayfurther comprise providing an optical amplifier configured to receivelight from the first light source and output an amplified light signalto the interferometer, and utilising the Amplified Spontaneous Emissionof the optical amplifier as the incoherent light source. The methodaccording to embodiments of this paragraph may further compriseoperating the interferometer in a third condition, in which the opticalamplifier receives an input light signal from the first light source andoutputs an amplified light signal to the interferometer; and correctingfor offsets and normalising the signals detected at the photodetectorsusing the determined normalisation factor. The first light source ispulsed and the method may further include determining a normalisationfactor and correcting for offsets and normalising the signals detectedat the photodetectors between pulses. The first light source maycomprise transmitted, reflected or backscattered light from an opticalfibre.

What is claimed is:
 1. An apparatus, comprising: a pulsed light source;an optical fibre deployed in an environment to be monitored and arrangedto receive pulses of light from the pulsed light source; aninterferometer arranged to receive light backscattered from along thelength of the optical fibre as the pulses of light travel along thefibre and to generate an interference signal in dependence thereon, theinterference signal comprising first, second, and third interferencesignal components having phase shifts therebetween; a firstphotodetector arranged to measure an intensity of the first interferencesignal component to provide first intensity data; a second photodetectorarranged to measure an intensity of the second interference signalcomponent to provide second intensity data; a third photodetectorarrange to measure an intensity of the third interference signalcomponent to provide third intensity data; and a processor timesynchronised with the pulsed light source and arranged to: i) receivethe first, second and third intensity data and to determine therefromany optical phase modulation in the received light; and ii) identifyacoustic perturbations incident along the optical fibre in dependence onthe determined optical phase modulation.
 2. The apparatus of claim 1,wherein the interferometer further comprises a fourth photodetectorarranged to monitor the intensity of the input light from the pulsedlight source.
 3. The apparatus of claim 1, comprising oversampling theoutputs of the photodetectors to provide multiple optical phase angledata over a spatial resolution of the apparatus.
 4. The apparatus ofclaim 3, wherein the outputs of the photodetectors are sampled at leasttwice over the spatial resolution.
 5. The apparatus of claim 3,comprising: determining a visibility factor from the combined outputs ofthe photodetectors at each sampled point; and providing a weightedsignal average of optical phase angle data from multiple sample pointsover the spatial resolution of the apparatus in dependence on thevisibility factor.
 6. The apparatus of claim 5, comprising: determininga visibility factor from the combined outputs of the photodetectors ateach sampled point; and adjusting the timing of the digitised samples ofthe photodetector outputs in dependence on the visibility factor.
 7. Theapparatus of claim 6, wherein the adjusting the timing of the digitisedsamples is performed by a digitiser with dynamically varying clockcycles.
 8. The apparatus of claim 1, wherein the pulsed light source isa plurality of laser light pulses with a plurality of wavelengths, andwherein the system further comprises wavelength division multiplexedcomponents used to multiplex the plurality of light pulses into theoptical fibre.
 9. The apparatus of claim 8, wherein the plurality oflight pulses are time shifted with respect to each other to control thecross-phase modulation between the plurality of light pulses.
 10. Theapparatus of claim 1, further comprising calibration circuitry arrangedto determine electrical offsets for the photodetectors during a firstcalibration condition in which no light propagates in theinterferometer.
 11. The apparatus of claim 1, further comprisingcalibration circuitry arranged to determine one or more of aphotodetector offset, a relative photodetector gain, and/or a couplingratio of optical paths in the interferometer from the outputs of thephotodetectors during a calibration condition in which an incoherentlight source inputs incoherent light to the interferometer.